In a mission to "improve the aesthetic appearance" of President Donald Trump's wall, active-duty troops will begin painting a stretch of border fence as part of a military deployment to secure the border at a time when tens of thousands of Central American families have been arriving in the U.S. and overwhelming the immigration system.

Border Patrol spokesman Carlos Pitones said troops on Friday will begin painting a one-mile section in Calexico, California, about 120 miles east of San Diego. The wall there has 30-foot-high rust-colored steel slats that were erected last year — the first border barriers built under Trump's presidency. Pitones declined to say which color they would be painted.

Sen. Dick Durbin called the wall-painting a "disgraceful misuse" of taxpayer money.

About 2,100 active-duty Marines and Army troops are on the border, along with an additional 1,900 National Guard members sent there by governors around the country. They are part of an aggressive use of Pentagon resources by Trump to secure the border, including his declaration of a national emergency that freed up billions of Defense Department dollars to build his long-promised wall.

The troops have been performing tasks such as installing concertina wire, providing logistical and helicopter support to Customs and Border Protection agents and monitoring motion cameras for signs of activity on the border. They cannot detain migrants or have much direct contact with them. If they encounter immigrants, they typically direct them to the nearest Border Patrol agent.

With the constraints, the troops have had little direct involvement in stopping the flow of migrants that is reaching new levels each month. Border Patrol agents made 132,887 apprehensions in May, including a record of nearly 85,000 adults in children. Photos of families waiting in jam-packed cells and in outdoor enclosures have generated outrage, and six children have died in the last year after being detained by border agents.

Department of Homeland Security said the primary purpose of the paint job is "to improve the aesthetic appearance of the wall" but noted that the new color could make it easier to spot migrants trying to blend in against the fence. The government also says a painted fence could make it harder for immigrants to scale, likely because the new coat made it more slippery.

The government made that determination after Customs and Border Protection earlier painted sections of barrier in Arizona. Some sections there were painted bright white.

Sgt. Dale Galloway, an artilleryman who has served tours in Afghanistan and the Middle East, has been serving on a surveillance team on the border since February in the active-duty deployment.

He said that when a record group of more than 1,000 migrants crossed the border recently, he was sitting in his truck monitoring infrared and optical cameras mounted on the bed.

"I saw it first," said Galloway. "My first thought was to contact CBP."

Galloway is not allowed to arrest migrants. He said that migrants have approached him about 10 times during his deployment. He directs them to the nearest Border Patrol agent so that they can turn themselves in.